The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor(s), to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Multiple Input Multiple Output (MIMO) systems in wireless communications leverage aspects of intersymbol interference to potentially increase the bandwidth efficiency of existing spectra. In wireless communication, radio waves do not propagate in a straight line between the transmitter and receiver, but rather bounce and scatter randomly off objects in the environment. This scattering, known as multipath, results in multiple copies of the transmitted signal arriving at the receiver via different scatter paths. MIMO leverages multipath to enhance transmission accuracy and allow multiple signals to be broadcast at the same frequency. This is done by treating the multiple scatter paths as separate parallel sub channels, each capable of bearing distinct data.
MIMO operates by splitting a discrete outbound signal into multiple substreams using an array of transmitter antennas to simultaneously launch the parallel substreams. In Orthogonal Frequency Division Multiplexing (OFDM), the outbound signal is de-multiplexed into a number of parallel spatial data streams or multiple copies of the outbound signal may be concurrently transmitted by the antennas in the same frequency band. All of the spatial data streams are transmitted in the same frequency band, so spectrum is efficiently utilized. Another array of antennas in the receiving device is used to pick up the multiple transmitted spatial streams and their scattered signals.
Each receive antenna picks up all of the incident transmitted spatial streams superimposed as observed components of the received signal vector, not separately. However, the multiple spatial streams are all scattered slightly differently, since the multiple spatial streams respectively originate from different transmit antennas located at different points in space. These scattering differences allow the spatial streams to be identified and recovered from the observed components of the received signal vector. MIMO in combination with OFDM constitutes the basis for many wireless communication standards, such as IEEE802.11n. Beamforming significantly improves the performance of MIMO-OFDM by spatially separating the transmitted data streams. With transmit beamforming, weights are applied to the transmitted signal to improve reception. The weights for each spatial stream, typically expressed as a steering matrix, are derived from channel state information (CSI) for each spatial stream.
Beamforming depends on weighting transmitted data streams according to the steering matrix. Calculating the steering matrix depends on having channel state information (CSI). Therefore, the channel needs to be sounded between two devices to measure the CSI. During sounding, device A transmits a packet to device B. Device B estimates the CSI from the Long Training Frame (LTF) in the packet preamble. One difficulty is that a poor channel between the devices may only support a single spatial stream prior to transmit beamforming, but the channel may support many spatial data streams with transmit beamforming. In a normal packet, the preamble contains information in the LTF for each spatial stream. Thus, if device A transmits a single stream packet to device B, the packet will only have information for one spatial stream. The full dimensionality of the channel is equivalent to the number of transmit antennas at device A and the number of receive antennas at device B. With information for only one spatial stream in the LTF, the CSI for only one spatial stream can be estimated. This limitation is addressed in wireless communication standard IEEE 802.11n.
IEEE 802.11n specifies the manner in which the CSI is communicated between devices. There are two approaches to channel sounding in the 802.11n standard, sounding using a null packet and sounding by way of Extension LTFs. The channel information in the LTF in the null packet preamble covers the full dimensionality of the channel. This type of packet contains no data, therefore there is no issue with a poor channel. In Extension LTF, the packet includes data, however, extra information in the LTF is included beyond that necessary for channel estimation of the data. The extra information in the LTF can be used to measure the full dimensionality of the channel (e.g., derive the CSI for each spatial stream).
Once the channel is sounded, channel information is fed back to the device that will be applying the steering matrix to beamform transmissions. 802.11n specifies two methods of feeding back the channel information: implicit feedback and explicit feedback. Implicit feedback is based on an assumption of reciprocity of the channel in both directions. Therefore, the CSI measured at either end of the channel is considered to be equivalent at the other end. The exchange of feedback and beamforming with implicit feedback per 802.11n is performed as follows. A device B sends a sounding packet (either a null packet or an extended LTF packet) to device A. Device A estimates the CSI from the LTF in the sounding packet. Device A computes the steering matrix from the estimated CSI. Then device A applies the weights in the steering matrix to the next transmission to device B.
With explicit feedback, the device performing the transmit beamforming is the same device as the device that transmits the sounding packet. Device A transmits a sounding packet to device B. Device B estimates the CSI from the LTF in the packet. Then device B transmits the estimated CSI or, in some cases, a steering matrix, to device A. Device A applies beamforming weights to the next transmission to device B.
While conventional Access Points (APs) are capable of beamforming, most conventional APs support only a single type of beamforming. However, APs serve many different types of wireless devices, which may not explicitly support beamforming or employ different types of beamforming methods.